Glycosylated hemoglobin concentrations were measured in 400 Pima Indians, aged 15 to 91 years, who were participating in the epidemiological studies of diabetes in the Gila River Indian Reservation in Arizona. At the same time plasma glucose levels, fasting and two-hours post-glucose load (75 g), were determined and compared with the hemoglobin to see which test might be the best indicator of the presence of diabetes. Previously the glucose levels, have been described by a model of two overlapping Gaussian distributions. Attemts were made to fit the same model to the glycosylated hemoglobin concentrations. In those aged 25 years and over, the glycosylated hemoglobins were biomodally distributed and like the glucose levels were a satisfactory fit to the overlapping Gaussian distribution model. These observations indicate the presence of two subpopulations in the Pima Indians, one normal and one diabetic. In preliminary studies, levels of glycosylated hemoglobin showed no better association with complications of diabetes than fasting and postload glucose levels.